Direct Electrical Heating (DEH) is a method for preventing wax and hydrates forming in subsea production pipelines of oil and gas. DEH is based on the fact that an electric alternating current (AC) in a metallic conductor generates heat in a single phase circuit, and DEH may be performed as follows. One cable is connected to the first end of the pipeline and a single core cable is piggybacked on (ie supported by) the pipeline and connected to the far end of the pipeline. The two cables together with the pipeline form a single phase electrical circuit. The single core piggyback cable is either strapped directly to the pipeline or located inside a mechanical protection profile which is strapped to the pipeline.
A traditional method for cable repair is to cut the cable subsea at the fault location, pull one end of the piggyback cable to the surface on a vessel and join the piggyback cable with an excess cable length stored on the vessel. The excess cable length is approximately 2.5 to 3 times water depth. The other end of the damaged cable is then pulled to the surface and dry spliced with the other end of the excess cable length. The piggyback cable is then re-installed on the pipeline with the excess cable length loop installed perpendicular to the pipeline. After electrical testing of the cable, the excess cable loop is rock dumped.
The following patent publications describe this existing technology:
EP1381117B1 (U.S. patent, filed 1982 Sep. 13, US NAVY)
U.S. Pat. No. 4,479,690 (European patent, filed 2003 Jul. 8, NEXANS)
There are some problems with the existing technology. With the current technology an excess length of 2.5 to 3 times the water depth is installed perpendicular to the pipeline and needs to be rock dumped. The pipeline also needs to be rock dumped in this area in order to avoid pipeline buckling. This is a time consuming operation with high cost. This operation requires a typical weather window of Hs<3 m in 24 hours.
For ultra deep water the excess cable length is up to 9 km which in some cases is longer than the length of the piggyback cable and pipeline. The total cost for a repair using existing technology can therefore be very high in ultra deep water.
In addition, the existing technology requires that the piggyback cable is able to carry its own weight at relevant water depth. This is not a challenge at 300-400 m water depth but for water depth in the area of 1000 m and deeper the copper conductor is not able to carry its own weight. This makes a repair scenario using existing technology very challenging.
The following documents also describe methods for repairing subsea cables. Chinese utility model CN 200949707Y (Shengli) describes a working cabin to allow maintenance of underwater cables without a surface boat. JP 4067711 (Hitachi) describes a capsule within which people may work underwater to cut a submarine cable. RU 2,336,196 (Uchrezhdenie) describes a compartment which allows personnel to work underwater. JP 10-145955 describes a container 102 filled with an insulating liquid 101 whose specific gravity is higher than water. Underwater cutting and connecting of cables 103 is performed in the liquid 101.